The field of the present invention is the treatment of wood by impregnation with a treating agent for the purpose of extending the useful life of the wood by the incorporation of a preservative therein.
The present invention relates to a process for the rapid fixing of heat-fixable wood preservatives such as CCA (copper, chromium, and arsenic oxides), CCB (copper, chromium, and boron oxides), ACA (ammoniacal or amine solution of copper and arsenic oxides), or the like, in a time period sufficiently short to be of practical value for modern high speed wood treaters.
The invented process greatly reduces and essentially eliminates drippage and subsequent environmental pollution from wood treated with chromated wood preservatives such as CCA. This process allows complete control of the fixation process, accomplishes the desired fixation in a short period of time, and further, does not delay modern or high speed wood treating operations. On the other hand, steam fixing takes several hours and air curing at ambient conditions takes days, weeks, or sometimes even months, for complete fixation to occur.
The most widely used wood preservative in North America is a mixture of copper, chrome and arsenic oxides, which is available in three types, denoted CCA-A, CCA-B and CCA-C. The C form is the one which has found widest acceptance in the United States. All three types are approved by the American Wood Preservers Association for use in the United States.
It has been established that to completely fix all of the copper, chrome and arsenic components present in CCA, they must be held at 70 F. for approximately 220 hours. Temperatures lower than 70 F. require even longer times.
Until complete fixation occurs, the treated wood is a potential source of environmental contamination. Rain water will wash or leach copper, chrome and arsenic out of the treated lumber and onto or into the soil, thus contaminating the soil over a period of time as the metals build up in the soil. Present EPA regulations define any soil or water that tests above 5.0 ppm for either arsenic or chromium as "hazardous waste", and the site at which they were found is designated "contaminated". Failing such a test results in a requirement for treaters and their customers to decontaminate the site where unfixed wood had been stored by removing and placing in approved hazardous landfills all contaminated soil, water, etc. until the site tests at one-hundredth of the failure level, or less than 0.05 ppm each of arsenic and chromium. From this it can be seen that failure by treaters or their customers to pass EPA tests over any area of consequence could result in economic charges that could be catastrophic. For example, CCA-C at 2.0 percent concentration, a concentration which many wood treaters approach or use in their daily operations, impregnates and coats the wood with up to 3,800 ppm of water-soluble arsenic (expressed as As.sub.2 O.sub.5). Many individual US treaters put more than a half-million pounds of 100% CCA per year into wood at their treating sites. As the average ambient temperature at which the treated wood is stored outdoors drops below 70 F. and approaches freezing, the time required for the components of CCA on and inside the wood to become water-insoluble becomes greater and approaches infinity. Thus, the value of this invention is readily apparent.
Many of the most successful wood treaters in the United States today are high-speed, high-volume producers. A typical treating cycle is one hour duration, and successful treaters seek to shorten this time. The most expensive individual piece of equipment in a wood treating plant is the pressure treating cylinder, which in an average modern plant is more than 6 feet in diameter and more than 65 feet long with quick-opening double doors, all of which are capable of withstanding treating pressures from full vacuum to over ten atmospheres. Increasing the time that the wood must remain in the pressure cylinder to any degree is very detrimental to the plant's return on investment (R.O.I.) For the same reasons additional pressure vessels are not desirable. What has long been sought is a process that does not add greatly to the capital cost of both existing and new treating plants. This process must also be capable of fixing the preservative by heating the wood so rapidly that the treated wood, in the same bundle as it comes out of the pressure treating cylinder, can be placed into the heating process without restacking. The entire heating portion of the invented fixing process takes no more, and preferably less, time than the pressure treating cycle, so that as each charge of treated wood emerges from the pressure cylinder, it can be immediately placed into the heating process and removed therefrom in time for subsequent charges, thus neither slowing nor interfering with established and proven procedures.
It is also important that the wood not be heated above the temperature which will cause deterioration. With steam at even slightly elevated pressures, destructive temperatures can be reached. In the present invention, using water alone without pressure, commercial wood species commonly treated with these chemicals cannot be overheated, and thus cannot be attacked by destructively high temperatures.
Applicant's initial work in this field was with steam as it is presently being practiced at some plants in Europe. The belief was held, by those highly experienced and skilled in the art of wood treating, that contact with quantities of water would wash active needed preservatives off the surface of the freshly treated wood, and the amount of preservatives, which becomes concentrated in the outer region of the wood, would be smaller in comparison with normally fixed timbers. (See Stanek et. al. U.S. Pat. No. 4,716,054, column 1, lines 37 -41). This is also the most likely reason that Kelso U.S. Pat. No. 4,303,705, requires heating under pressure with the treating solution containing wood treating chemicals in his one example of aqueous (non steam) heating in column 4. When utilizing this Kelso procedure, it has been found that the quantity of hazardous waste formed by heating with the treating solution is so great as to be undesirable, as well as uneconomic, particularly with today's increasingly stringent environmental requirements.
The American Wood Preservers Association requires that certain chemical retentions be present in the outer layers of the wood. Therefore, it is important to secure quick-fixation without depleting the wood of the required content or level of preservative in its outer layer.
A minor, but not insignificant need, in treated wood is for the treated wood to have some color difference from untreated wood. Present U.S. practice, which is almost exclusively based on allowing the treated wood to stand in open areas, results in vastly differing shades ranging from bright green to almost brown, depending on temperature and solar exposure. This extreme color variation is unsightly and visually detrimental for many applications such as decks, fences, and many other unpainted applications. The invented process produces a much more uniform wood surface color.
It is preferable not to heat the pressure cylinder, because the heavy cylinder walls act as a heat sink. The heat thus contained accelerates the reaction of the CCA solution that is later charged into the cylinder to treat subsequent charges of wood, causing the CCA to react prematurely with extracted reducing sugars, etc., and with small particles of wood that are associated with wood treating solutions. This is not only uneconomic causing premature precipitation of the CCA, but can also be a major generator of hazardous wastes. This is particularly true when processes are used that increase the extractables in the CCA-containing wood treating solution, such as modified full-cell or modified empty-cell processes.
Normally, wood is treated while "bundled", that is, stacked and banded. "Stickering" is the act of placing thin strips or pieces of wood between each layer of lumber, which requires unbundling to accomplish. Present European practice for the heat fixing of CCA-treated wood by steam requires unbundling or stickering of the treated wood in order to reduce steaming time from about five hours to two and a half hours. Such stickering would be highly undesirable in the U.S.A., because it requires increased handling of the wood after treatment. If stickering is done before treatment, the extra spacing between the layers reduces the capacity of the pressure cylinder.
When preservative-impregnation is accomplished by high pressure, followed by a vacuum, the action of preservative chemicals returning to the treating solution during the low pressure phase is called "kickback".
Another environmental problem faced by wood treaters is that present treatment practice often leaves surface deposits of the treating chemicals in various stages of reaction that are easily washed off onto or into the soil by later rainfall, where they contaminate the environment.